As positive-working photoresist compositions, those compositions which contain an alkali-soluble resin and a light-sensitive naphthoquinonediazide compound are generally used. For example, compositions containing a "novolak phenol resin/naphthoquinonediazide-substituted compound" are described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470, etc., and examples of a "novolak resin composed of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfon ic acid esters" are described in L. F. Thompson, Introduction to Microlithography (published by ACS, No. 219), pp. 112-121.
The novolak resin used as a binder is particularly useful since it is soluble in an alkali aqueous solution without swelling and shows a high resistance particularly against plasma etching when using an image formed therein as a mask for etching. The naphthoquinonediazide compound used as a light-sensitive material decreases alkali solubility of the novolak resin and, when decomposed by irradiation with light, produces an alkali-soluble substance and actually increases the alkali solubility of the novolak resin. The naphthoquinonediazide compound is particularly useful because of this great change in its properties induced by light.
Many positive-working photoresists containing the novolak resin and the naphthoquinonediazide light-sensitive material have so far been developed and put into practice in view of the above-described advantages, and sufficient results have been obtained in forming lines with a width of about 1.5 .mu.m to about 2 .mu.m.
However, the degree of integration of integrated circuits is increasing more and more and, in manufacturing semiconductor bases such as VLSI, formation of ultramicro-patterns with a line width as narrow as 1 .mu.m is required. For such uses, the conventional positive photoresists described above are insufficient, and photoresists having a high resolving power, an ability to accurately reproduce a pattern of an exposure mask, and a high sensitivity for attaining high productivity are required.